Engine performance immediately after start-up can be improved by diverting exhaust gases from the exhaust manifold to an area around the carburetor for heating the intake air, thus permitting improved vaporization of the fuel-air mixture. It has been a common practice to utilize a three-port valve for valving an intake manifold vacuum source to a vacuum motor which opens a heat valve enabling exhaust gas heat to be transferred to the carburetor. As the engine continues to operate, the exhaust gas temperature approaches a level which can result in excessive heating of the incoming air to the carburetor leading to vapor lock. Before this point is reached, the three-way valve shuts off the vacuum motor by isolating the vacuum source and valving the motor to a venting port. Prior art devices in common use for performing these valving functions include the standard three-port spring biased ball-type valve. Valve chatter is a problem encountered with ball-type three port valves. During valving of the vacuum motor port between the vacuum source and the venting source, there is a point at which the vacuum source is fluidly connected to the venting port resulting in a sudden pressure build-up around the valve ball which tends to lift the ball off its seat until overcome by the return spring. As the spring returns to its normal position, the fluid pressure again lifts the ball off its seat setting up the chattering effect until the ball seals off the vacuum source. A further problem associated with the use of a standard three-port valve is that it has been necessary to utilize an independently mounted check valve located between the vacuum port of the three-way valve and the vacuum source in order to isolate the vacuum motor from fluctuations in the vacuum source caused by sudden vehicle accelerations. A separately mounted check valve increases the total unit manufacturing costs plus adds to installation costs.